As a technology close to the present invention, the vehicle electronic control device as disclosed in Japanese Patent Application Laid-open No. S60-11753, is known. This control device includes, for example, an input shaft rotation sensor provided on the input shaft side of a transmission; a vehicle velocity sensor provided on the output shaft side of the transmission; a signal processing device that receives the output from each sensor and obtains the speed of rotation of the input shaft and the output shaft of the transmission; and a gear change ratio measurement device that determines the gear change ratio of the transmission. Also, when the gear change ratio measurement device detects that the gear change ratio is outside the mid position, and the output of one sensor indicates that the speed of rotation is greater than a predetermined value, and the speed of rotation output from the other sensor is low, the control device determines that the sensor with the low output has a breakdown. The control device then estimates the output value of the sensor with the breakdown from the output of the sensor without the breakdown and the gear change ratio measured by the gear change ratio measurement device. By using the estimated sensor output value in the transmission control in this way, the transmission operation enables safe driving to be carried out.
There are electronic controlled four-wheel drive vehicles having drive torque estimation devices including first drive torque calculation means, second drive torque calculation means, slip ratio calculation means, and torque combination means. The first drive torque calculation means calculates the estimated drive torque of the engine based on the amount of air taken into the engine. The second drive torque calculation means calculates the estimated drive torque of the engine based on the speed of rotation of the engine output shaft. The slip ratio calculation means calculates the slip ratio of the torque converter. The torque combination means calculates the combination drive torque by combining the first estimated drive torque calculated by the first drive torque calculation means and the second estimated drive torque calculated by the second drive torque calculation means in a predetermined proportion. In most operating conditions, the estimation accuracy of the first estimated drive torque is good. However, during vehicle start up (in other words, when the slip ratio of the torque converter is low) the estimation accuracy is not so good.
Therefore when the slip ratio of the torque converter is equal to or less than a predetermined value, the drive torque estimation device calculates the engine drive torque as the combination drive torque from the predetermined combination of the first estimated drive torque and the second estimated drive torque. Also, the distribution ratio of drive power to the front and rear wheels of the four-wheel drive vehicle is determined based on the highly accurate estimated drive torque calculated by the drive torque estimation device.
However, in the drive torque estimation device as described above, if there is a breakdown in the speed of rotation measurement sensor on the output shaft of the torque converter (in other words, the main shaft, which is the input shaft to the transmission) due to a broken cable or similar, the slip ratio of the torque converter will not be calculated correctly. For example, the slip ratio of the torque converter that is the subject of the calculation could be a constant zero. At this time the drive torque estimation accuracy becomes worse, as the second estimated drive torque is always included in the engine estimated drive torque, so driving safety could be reduced.